Although many gastrointestinal (GI) disorders are intimately related to cellular relationships established during embryogenesis, current understanding of GI developmental mechanisms is incomplete. GI tract organogenesis requires exchange of signals between cells derived from gut-specific endoderm and the adjacent splanchnic mesoderm; investigating the nature and mechanisms of this exchange is important. Among the handful of pathways that mediate virtually all developmental signaling, the role of Hedgehog (Hh) proteins is especially prominent in the gut. Two Hh ligands, Sonic (Shh) and Indian (Ihh) are highly expressed early in fetal gut endoderm, from where they signal to the adjacent mesenchyme. Disruption of either gene in mice results in specific but surprisingly limited GI developmental anomalies; because redundancy between the two Hh factors may restrict the scope of defects, we studied mice lacking both Hh factors in gut endoderm. Shh/Ihh-double mutant mice have profoundly abnormal digestive tract development as a result of marked attrition of GI mesenchymal cells. In Specific Aim 1 we propose to define the roles and mechanisms for Hh signaling in early GI development. First, we will dissect the relative importance of Hh signaling in proliferation, survival and differentiation of stomach and intestinal mesenchymal cells, especially with respect to smooth muscle lineages. Second, our observations in mouse embryos that lack all Hh signaling agree with ideas previously proposed in the literature: that Hh signals induce expression of the secreted factor Bone morphogenetic protein (BMP)-4 and transcription factors FoxF1 and FoxF2. We propose experiments to determine to what extent these putative downstream targets of Hh signaling are responsible for the global effects on GI organogenesis. The stomach anomalies in Shh/Ihh-double null embryos are identical to those we identified in transgenic mice where the Notch signaling pathway is aberrantly activated in fetal stomach mesenchyme. Other preliminary data also support the novel hypothesis to follow from these findings: that Hh signaling in fetal stomach functions in part to restrict mesenchymal Notch activation and hence prevents Notch- induced cell death. Specific Aim 2 outlines a series of in vivo and ex vivo studies to test this hypothesis critically, making particular use of several mouse genetic models to define the nature and extent of interactions between Hh and Notch signaling in GI organogenesis. Although the available experimental models permit incisive experiments in stomach development, similar principles are likely to apply throughout the digestive tract. The studies we propose address important outstanding questions, explore developmental signaling and regulatory mechanisms in molecular detail, and will contribute toward improved understanding and treatment of common GI disorders. PUBLIC HEALTH RELEVANCE. One particular surprise from the last decade of developmental biology research is that complex developmental processes in diverse tissues depend on just a few cell signaling pathways. The hedgehog pathway plays a vital role at many steps in development and homeostasis of the digestive tract and is also implicated in cancers of endodermally derived tissues. The functions and mechanisms of hedgehog signaling in gut development are poorly understood, in spite of their useful implications and applications for human gastrointestinal disorders. This proposal seeks to apply genetic and cell culture tools to elucidate the precise role and mechanisms of hedgehog signaling in genesis of the mouse stomach and intestine.